Method for the production of a piston for an internal combustion engine

ABSTRACT

A method for the production of a piston made of steel, for an internal combustion engine, in which the upper piston part is produced using the forging method, and the lower piston part is produced using the forging or casting method, and they are subsequently welded to one another. To simplify the production method and make it cheaper, the upper piston part is forged using the method of hot forming and of cold calibration, to finish it to such an extent that further processing of the combustion bowl and of the upper cooling channel regions can be eliminated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 14/576,384,filed on Dec. 19, 2014, now U.S Pat. No. 9,339,898, which is adivisional of U.S. patent application Ser. No. 13/066,559, filed on Apr.18, 2011, now U.S Pat. No. 8,943,687, which claims priority under 35 USC119 from DE 10 2011 013 141.8, filed on Mar. 4, 2011, the disclosures ofwhich are herein incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for the production of a piston for aninternal combustion engine.

From the state of the art, it is generally known to produce pistons fromsteel for an infernal combustion engine, in that first an upper pistonpart is produced using the forging method, and a lower piston part, isproduced using the forging method or by means of casting, and then theupper piston part is welded to the lower piston part. In this regard,reference should be made to the patent documents DE 195 01 416 A1, DE-OS29 19 638, DE 196 03 589 A1, and DE 198 46 152 A1. In this connection,the method of hot forming, in other words hot forging, at a steeltemperature of 950° C. to 1300° C, is used.

In this connection, an uncontrollable oxide layer forms on the surfaceof the forged blank, and in order to remove it, the surface of theforged blank must be blasted with coarse blasting material. This resultsin great variations in the forged contour, so that as a consequence ofthis, complicated reworking of the forged blank, by means of achip-cutting processing method, is required,

SUMMARY OF THE INVENTION

Accordingly, it is the task of the present invention to avoid theaforementioned disadvantages of the state of the art, whereby inparticular, complicated reworking of the combustion bowl and of thecooling channel is supposed to be avoided.

It is furthermore the task of the present invention to indicate a methodwith which pistons having combustion chamber bowls and cooling channelsthat are not configured with rotation symmetry or in centered manner canbe produced in cost-advantageous manner.

Finally, it is the task of the present invention to indicate a methodwith which pistons can be produced, in which the wall between the edgeof the combustion bowl and the upper part of the cooling channel has aconstant thickness over the circumference.

These tasks are accomplished with the characteristics that stand in thecharacterizing part of the main claim and of the dependent claims.Advantageous embodiments of the invention are the object of thedependent claims.

In this connection, the result is achieved, by means of cold calibrationor cold forming of the forged blank, that the combustion bowl and thecooling channel are formed in finished manner.

BRIEF DESCRIPTION OF THE DRAWINGS

Some exemplary embodiments of the invention will be explained in thefollowing, using the drawings. These show:

FIG. 1 a sectional diagram of a piston produced according to the methodaccording to the invention, in a section plane that lies perpendicularto the pin bore axis,

FIG. 2 a section through the piston, in a section plane that, lies onthe pin bore axis,

FIG. 3 a section through the upper piston part after semi-hot forming,

FIG. 4 a section through the upper piston part after over-lathing of theouter contour and of the contact regions intended for friction welding,

FIG. 5 a top view of a configuration of the upper piston part having anasymmetrically configured and eccentrically disposed combustion bowl,

FIG. 6 a section through the upper piston part along the line VI-VI inFIG. 5,

FIG. 7 the upper piston part and the lower piston part before joining bymeans of friction welding,

FIG. 8 a top view of another embodiment of the upper piston part havingan asymmetrically configured and eccentrically disposed combustion bowland having a valve niche, and

FIG. 9 a section through the upper piston part along the line IX-IX inFIG. 8.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows an embodiment of a piston 1 produced according to themethod according to the invention, in section, perpendicular to the pinaxis 2, consisting of an upper piston part 3 and a lower piston part 4,which are connected with one another by way of a friction-welding seam5.

The piston 1 has a piston crown 6 into which a combustion bowl 7 isformed. Radially on the outside, a ring wall 8 directed downward, havinga ring belt 9 for piston rings not shown in the figure, is formed ontothe piston crown 6. Radially within the ring wall 8, the piston 1 has aring-shaped support 10 formed onto the underside of the piston crown 6.

The lower piston part 4 consists of two skirt elements 11 and 12 thatlie opposite one another, which are connected with one another by way oftwo pin bosses 13 and 14 that lie opposite one another, each having apin bore 15 and 16. In FIG. 1, only the pin boss 13 having the pin bore15 can be seen, because of the position of the section plane.

A ring-shaped contact part 17 connected with the pin bosses 13, 14 isdisposed on the top of the lower piston part 4. Furthermore, the lowerpiston part 4 has a circumferential ring rib 18 on its top, which rib isdisposed radially outside of the contact part 17 and connected with theskirt elements 11, 12. A radially oriented ring element 19 extendsbetween the contact part 17 and the ring rib 18.

In this connection, the support 10 and the contact part 17 are disposedin such a manner that the underside of the support 10 and the top of thecontact part 17 have contact with one another and form a first contactregion 20. Furthermore, the ring wall 8 and the ring rib 18 are disposedin such a manner that the lower face side of the ring wall 8 and the topof the ring rib 18 also have contact with one another and form a secondcontact region 21. The first and the second contact region 20 and 21form friction-welding surfaces during the production of the piston 1.

In this way, the result is achieved that a circumferential coolingchannel 22 disposed close to the piston crown 6, radially on theoutside, is delimited, at the top, by the piston crown 6, radially onthe inside partly by the piston crown 6, partly by the support 10, andpartly by the contact part 17, at the bottom by the ring element 19, andradially on the outside partly by the ring wall 8 and partly by the ringrib 18. The cooling channel 22 has an inflow opening for introduction ofcooling oil and an outflow opening for discharge of cooling oil, butthese are not shown in the figure.

In FIG. 2, the piston 1 is shown in section along the pin bore axis 2.Here, the two pin bosses 13, 14 can be seen, with the contact part 17formed onto them, as can the ring element 19 that is connected with thecontact part 17 and the pin bosses 13, 14, respectively.

The piston 1 is produced from AFP steel, in other words fromprecipitation-hardened ferritic-pearlitic steel, such as case-hardenedsteel 38MnVS6, for example. However, any other suitable steel can beused, such as tempered steel 42CrMo4, for example. In this connection,production of the lower piston part 4 takes place in conventionalmanner, by means of casting or hot forging.

The upper piston part 3 is produced by means of the method of hotforming. In this connection, a piece of AFP steel that is shaped to fitinto the drop-forging machine intended for the upper piston part 3 isheated to 1200° C. to 1300° C., and subsequently formed or pre-formed inmultiple forming stages, in other words forging processes, in the samedrop-forging machine. The scale that forms during forging is removed bymeans of blasting.

Subsequently, the finished forged upper part blank is cold-calibrated atroom temperature, whereby all the surfaces of the upper piston part 3are pressed at room temperature, in order to achieve the finaldimensions.

Alternatively to this, the pre-formed upper part blank can also bebrought into its final shape by means of cold-forming at roomtemperature. It is advantageous, in this case, if an annealing processis still carried out before blasting, in order to reduce the tendency toform cracks during cold forming.

Furthermore, other processes can also be used for production of thepre-form, such as the method of cold forming, of semi-hot forming, or ofmilling, for example. Thus, the pre-form can also be produced by meansof a precision casting method. In order to avoid scale formation, thelatter method should be used under an inert gas atmosphere.

The resulting blank of the upper piston part 3 is shown in FIG. 3. Inthis connection, the combustion bowl 7, the upper cooling channelregion, and the inner mandrel region 29 are already formed in theirfinal form, so that no further processing steps are any longer requiredin these regions. In this connection, the result is also achieved thatthe wall thickness between the bowl edge and the upper cooling channelregion is almost constant over the circumference. The upper piston part3 as it looks after finishing is shown in FIG. 3 with broken lines.

In the subsequent method step, the radially outer region 23 of thepiston crown 6, the radially outer region 24 of the upper piston part 3intended for the ring belt 9, the lower face surface 25 of the ring wall8, the lower region 26 of the inner surface 27 of the ring wall 8, andthe contact surface 28 of the support 10 are machined by means oflathing, so that the upper piston part 3 as shown in FIG. 4 is obtained.The lower region of the cooling channel 22, the lower face surface 25 ofthe ring wall 8, and the contact surface 28 of the support 10 are formedin finished form after this latter method step. Here again, the upperpiston part 3, as it looks after finishing, is shown with broken lines.

The production method of hot forming in combination with coldcalibration or cold forming, respectively, particularly allowsproduction of upper piston parts 3′ having combustion bowls 7′ that areconfigured asymmetrically and disposed eccentrically, as shown in FIG. 5and 6. Here, again, no further processing of the combustion bowl 7′ isrequired any longer, once the process of hot forming and of coldcalibration or cold forming, respectively, of the upper piston part 3′has been completed.

In the present exemplary embodiment according to FIG. 5 and 6, thecombustion bowl 7′ has approximately the shape of a four-leafed clover.However, any desired shape of a combustion bowl can be implemented withthe method of hot forming in combination with cold calibration or coldforming, respectively.

FIGS. 8 and 9 show the upper piston part according to FIG. 5 and 6,produced in this manner, whereby in addition, a valve niche 30 has beenformed into the piston crown 6 of the upper piston part 3″.

The upper piston part 3, 3′, 3″ according to FIG. 4, 5, 6, 8, 9 isbraced into a friction-welding device (not shown in the figure) togetherwith the lower piston part 4, and, as shown in FIG. 7, they are broughtinto position, relative to one another, so that they can be put intorotation, moved toward one another with force, and friction-welded toone another when the upper piston part 3, 3′, 3″ makes contact with thelower piston part 4 in the region of the contact regions 20 and 21. Ifthe combustion bowl 7′ is configured asymmetrically or eccentrically,care must be taken during friction welding to ensure that aftercompletion of the welding process, the combustion bowl 7′ assumes aclearly defined rotation position relative to the pin axis 2, forexample.

In this connection, the piston 1 shown in FIGS. 1 and 2 is obtained.

Within the scope of the last method step, the grooves of the ring belt 9are lathed into the outer piston wall and the piston crown 6 is lathedflat, as indicated in FIGS. 3 and 4. Furthermore, the precision pistoncontour and the pin bores are worked in.

REFERENCE SYMBOL LIST

-   1 piston-   2 pin axis-   3, 3′, 3″ upper piston part-   4 lower piston part-   5 friction-welding seam-   6 piston crown-   7, 7′ combustion bowl-   8 ring wall-   9 ring belt-   10 support-   11, 12 switch element-   13, 14 pin boss-   15, 16 pin bore-   17 contact part-   18 ring rib-   19 ring element-   20 first contact region-   21 second contact region-   22 cooling channel-   23 outer region of piston crown 6-   24 outer region of upper piston part-   25 lower face surface of ring wall 8-   26 lower region of inner surface 27 of ring wall 8-   27 inner surface of ring wall 8-   28 contact surface of support 10-   29 inner mandrel region-   30 valve niche

What is claimed is:
 1. A method for the production of a piston for aninternal combustion engine, comprising the following steps: producing anupper piston part made of steel by forging, said upper piston parthaving a piston crown having a combustion bowl, a ring wall formed ontothe piston crown radially on the outside, directed downward, and aring-shaped support disposed radially within the ring wall formed ontoan underside of the piston crown, wherein an upper part of a coolingchannel is formed between the ring wall and the support, producing alower piston part made of steel by forging or casting, said lower pistonpart having two skirt elements that lie opposite one another, which areconnected with one another by way of two pin bosses that lie oppositeone another, a ring-shaped contact part disposed on a top of the lowerpiston part and connected with the pin boss, and a circumferential ringrib disposed radially outside of the contact part and connected with theskirt elements, wherein a lower part of the cooling channel is formedbetween the contact part and the ring rib, welding the upper piston partto the lower piston part by way of contact surfaces of the ring wall andthe ring rib, and of the support and the contact part, respectively,wherein the cooling channel formed by the upper piston part and by thelower piston part is closed, and finishing the piston using achip-cutting production method, wherein during the step of producing theupper piston part, an upper piston part blank is pre-formed using thehot-forming method, at 1200° C. to 1300° C., after which the upperpiston part blank is cold-formed at 0° C. to 150° C., after which thecombustion bowl and/or the upper part of the cooling channel undergo nofurther processing, and subsequently a radially outer region of thepiston crown, a radially outer region of the ring wall, a lower regionof an inner surface of the ring wall, and the contact surface of thesupport of the upper piston blank are finished to produce the upperpiston part.
 2. A method for the production of a piston for an internalcombustion engine, comprising the following steps: producing an upperpiston part made of steel by forging, the upper piston part having acombustion bowl, a ring wall formed onto the piston crown radially on anoutside, directed downward, and a ring-shaped support disposed radiallywithin the ring wall formed onto an underside of the piston crown,wherein an upper part of a cooling channel is formed between the ringwall and the support, producing a lower piston part made of steel byforging or casting, the lower piston part having two skirt elements thatlie opposite one another, which are connected with one another by way oftwo pin bosses that lie opposite one another, a ring-shaped contact partdisposed on a top of the lower piston part and connected with the pinboss, and a circumferential ring rib disposed radially outside of thecontact part and connected with the skirt elements, wherein a lower partof the cooling channel is formed between the contact part and the ringrib, welding of upper piston part to the lower piston part by way ofcontact surfaces of the ring wall, and the ring rib, and of the supportand the contact part, respectively, wherein the cooling channel formedby the upper piston part and by the lower piston part is closed, andfinishing the piston using a chip-cutting production method, whereinduring the step of producing the upper piston part, an upper piston partblank is cold-formed at 0° C. to 150° C., after which the combustionbowl and/or the upper part of the cooling channel undergo no furtherprocessing, and subsequently a radially outer region of the pistoncrown, a radially outer region of the ring wall, a lower region of aninner surface of the ring wall, and the contact surface of the supportof the upper piston blank are finished to produce the upper piston part.